Preheat oven to 180 C temp. Grease the edges of 22 cm round cake pan, line the bottom with parchment paper.

In a medium-size bowl, combine flour, baking powder, salt.

In a big bowl, using the electric mixer, beat the butter and sugar until smooth and creamy.

Add the eggs one at a time and lightly whisk.

Add the buttermilk and vanilla extract. Whisk until smooth.

Add the flour mixture and mix with a spatula until just combined. Spread the batter into a pan.

Lighty whisk the mascarpone, egg, sugar and flour. Spread on the batter top. Drop the blueberries.

In a small bowl, add all streusel ingredients. Combine with a fork or hands until forms small pieces. Drop on the cake top.

Bake about 50-55 min. Remove from a pan when completely cool.